Infectious Bronchitis Virus (IBV) is a Corona virus which is infectious to birds and in particular to poultry. Infection with the virus results in an acute respiratory disease, characterized by trachea rales, coughing, sneezing and nasal discharge. Infectious bronchitis (i.b.) may cause a considerable mortality among afflicted chickens, and moreover may damage their kidneys. In layer and in breeder hens the infection may cause a drop in egg production due to damage of the reproductive tract. In many cases the egg drop is accompanied by an enteritis causing diarrhea.
Poultry can be protected against IBV infections by vaccination. Moreover, there is a desire to protect the birds at as early an age as possible. These young chickens are also vulnerable to infection by other viruses, such as Newcastle Disease Virus (NDV), Infectious Bursal Disease Virus (IBDV), Marek's Disease Virus (MDV), other avian Herpes Viruses, Fowl Pox Virus (FPV), Avian Encephalomyelitis Virus (AEV), Reticulo-endoteleosis Virus (REV), avian Adeno Viruses and avian Reo Viruses. Of course, there is a desire to protect the young chickens not only against IBV infection but to protect them against infections by other of the above-mentioned viruses as well. To this end, young chickens have been vaccinated with vaccines comprising combinations of immunogens derived from IBV as well as from at least one of the other viruses. These vaccines may contain live or inactivated viruses, but in principle live vaccines are favored. A well known problem with these live combination vaccines is the mutual influence of the antigenic components resulting in a decrease of the potency of one or more of the constituting components. This problem has been reported for the combination IBV/NDV by e.g. Raggi & Lee (1964) Avian Disease 8, 471-480; Hanson & Alberts (Am.J.Vet.Res. March 1959, 352-356) and Thornton & Muskett (The Veterinary Record, May 1975, 467-468); conflicting results were published by Winterfield (Poultry Science 1984, 63, 182-184).